We have demonstrated that ovaries taken late in gestation from fetal rhesus monkeys form 3 times more estradiol from C14 progesterone (P) than do fetal testes or ovaries from a 94-day-old fetus. The capacity to form estrogen correlates with the appearance of medium-sized follicles in the fetal ovary. One animal whose brain and pituitary were removed on day 100 showed neither follicular growth nor estradiol-beta17 (E2) biosynthesis in vitro at 150 days. These data suggest the fetal pituitary plays a role in regulating estrogen biosynthesis by the fetal ovary. P administered to the fetus or added to testicular incubations always stimulated testosterone formation. We compared the relative potency of testosterone (T) or E2 to supress LH and FSH in long- and short-term castrated monkeys (males, females and androgenized females) to determine the effects of prenatal androgen on this process. T was unable to suppress gonadotropins in this species even at higher than physiological levels. E2, however, quickly (within 6 hrs) and efficiently suppressed LH and FSH in both sexes. BIBLIOGRAPHIC REFERENCES: Resko, J. A., 1975. Sex steroids in the circulation of the fetal and neonatal rhesus monkey: A comparison between male and female fetuses. INSERM 32: 195-204. Resko, J.S., 1975. Estrogens in fetal and maternal plasma of the rhesus monkey. Endocrinology 97: 425-430.